Plastic materials have been used for the manufacture of ophthalmic lenses for many years. Plastics offer advantages to the patient over glass, most noticeably in their lower densities, allowing for a lighter lens, and greater impact strength. Conversely, plastic lenses can exhibit disadvantages: they tend to scratch more easily, have higher levels of chromatic aberration (lower ABBE values), and may distort at higher temperature processing conditions, due to lower glass transition (Tg) temperatures, resulting in "warped" lenses, or lenses with high levels of optical distortion. Additionally, plastic lenses usually have lower index of refraction values when compared to glass, which tends to require lenses of increased thickness and reduced cosmetic appeal. The standard "CR-39" type lens, made from diethylene glycol bis(allyl carbonate), has an index of refraction of 1.498.
Advances in technology have allowed improvements in plastic lens performance. Coatings have been developed which impart improved scratch resistance. Some plastics have relatively high ABBE values which are adequate for minimizing the effects of chromatic aberration. Improvements in machining and optical lens processing equipment and processes have permitted the use of materials having lower glass transition temperatures. Plastics with higher indices of refraction, and physical lens design improvements, have helped improve the cosmetic appeal of plastic lenses.
While these improvements have helped the plastic lens gain acceptance in the marketplace, to a point where plastic lenses constitute a majority of lens eyewear in the U.S., increasing expectations for performance have dictated that good ophthalmic lenses have the following characteristics:
Clarity and Color
Lenses which are "hazy" are obviously unacceptable to the patient. Color is also of concern both from the standpoint of having a "water-white" quality for best cosmetic appeal when the lens is first purchased, as well as maintenance of that color over time, e.g., being resistant to the effects of sun light, which over time can cause the lens to turn yellow.
Optical Distortion
Lack of optical distortion requires a material having a relatively high ABBE number such that chromatic aberration is minimized; and, more importantly, a material having excellent uniformity in composition such that the occurrence of visible "waves" is minimized.
Rapidly Tintability
Many ophthalmic lenses are manufactured in semi-finished form and shipped to optical laboratories where the prescription is "ground in". With ever increasing emphasis on short optical lab turn-around times, e.g., 1 hour service, the ability of the lens material to rapidly accept fashion tints is important. In the case of semi-finished lens products, the front surface of the lens may have a scratch resistant coating which does not accept tint. Thus, the only route for tinting to occur may be the parent lens material on the back surface. This material must be tintable.
High Index of Refraction and Low Density
The higher the index of refraction, the thinner the finished lens will be for a given design. This higher index, especially when combined with a relatively low density, will allow for the manufacture of "thinner and lighter" lens products.
Over the past several years, plastic ophthalmic lenses have been fabricated from a variety of materials including polycarbonate and polymethylmethacrylate, as well as polymerized allylic compounds, epoxies, and urethanes. The most common plastic ophthalmic lens, however, is made from diethylene glycol bis(allyl carbonate) often referred to as "CR-39" (a specific product manufactured by PPG Industries). As previously mentioned, this material has a refractive index of 1.498. It is easily processed in optical laboratories, is able to be manufactured with low optical distortion and is readily tinted by various commercially available tinting dyes.
The use of polyester materials to produce ophthalmic lenses has been previously disclosed in various U.S. patents. Examples of such disclosures are U.S. Pat. Nos. 3,391,224 and 3,513,224. U.S. Pat. No. 3,391,224 discloses a composition in which a polyester is combined with from 5 to 20 weight percent methyl methacrylate and less than 5 weight percent styrene to produce a thermosetting product which can be used to produce an ophthalmic lens. U.S. Pat. No. 3,513,224 discloses a composition in which 70 to 75 weight percent of a specific unsaturated polyester formed from the reaction of fumaric acid with triethylene glycol and 2,2-dimethyl-1,3-propanediol (otherwise known as neopentyl glycol) is combined with about 12 to 18 weight percent styrene and 8 to 12 weight percent ethylene glycol dimethacrylate. The styrene raises the index of refraction to approximately 1.52, and the ethylene glycol dimethacrylate reduces brittleness of the polymer.
A number of commercially available unsaturated polyester resins has been developed which are clear when cast and have a refractive index of approximately 1.56 (the high index being primarily attributable to the use of styrene as a cross-linking diluent monomer at a level of approximately 30 to 45 weight percent). The densities of the various polyester systems are also quite low (on the order of 1.25 grams/cc). These properties are superior to CR-39 (index of 1.498 and density of 1.32 grams/cc) with regard to the potential to make "thinner and lighter" lenses.
Polymerization of the polyester resin system can be carried out in a number of ways. Quite common is the use of a system promoted with a material such as cobalt octoate or cobalt naphthenate. When used with methyl ethyl ketone peroxide, the system can be cured near room temperature. Other free radical polymerization techniques also can be used, including thermal curing using peroxides or diazo compounds, as well as photoinitiated curing using compounds selected from the following classes of photoinitiators: benzoin ethers, benzophenones, thioxanthones, ketals, acetophenones, and phosphine oxides.
Polyester resins can be manufactured using different compositions to achieve a wide variety of physical properties (hard, soft, rigid, flexible, and the like). Typical commercial polyesters include those made from a variety of glycols and acids. Common glycols used in alkyd polyester synthesis include: ethylene glycol, diethylene glycol, propylene glycol, dipropylene glycol, neopentyl glycol, and the like. Common acids used include: phthalic anhydride, isophthalic acid, adipic acid, and the like, used in conjunction with maleic anhydride and/or fumaric acid to provide unsaturation for cross-linking, usually with styrene or other diluent monomers.
Resins made using phthalic anhydride are commonly called "ortho resins"; those made with isophthalic acid are commonly referred to as "iso resins". With respect to properties desirable for making ophthalmic lenses, typical iso resins which have good scratch resistance are generally quite slow to tint. Typical ortho resins, on the other hand, are generally more scratch-prone, but tint more readily. All of the unsaturated polyester resins have a propensity to polymerize somewhat non-uniformly causing internal optical distortion or visible "waves".
As previously mentioned, styrene is commonly used as a cross-linking diluent monomer with unsaturated polyester compositions. As the portion of styrene is increased, the index of refraction also increases. However, the presence of higher concentrations of styrene also tends to cause a higher degree of exotherm within the polymerizing composition leading to the formation of optical distortion within the lens. Improving optical uniformity (lowering optical distortion) and increasing tint speed in an unsaturated polyester resin-based ophthalmic lens composition, while maintaining a high index of refraction and color, therefore, is a formidable task.